Many technologies for locating mobile units have been developed; particularly in response to recently enacted legislation regarding positioning in the event of an emergency. Wireless carriers are required to provide the location of an emergency caller, (such as a 911 caller), for public safety purposes.
In addition to emergency services, numerous geolocation-based applications are envisioned for the future. Among them are network optimization, where spatio-temporal wireless traffic distribution is used in planning, design, and operation of wireless networks; information services such as providing mobile users with directions to gas stations, restaurants, hotels or the like; tracking of personnel, vehicles, and assets in managing company operations; and location-sensitive billing of mobile users to enable more precise rate structures for wireless service providers.
One widely used geolocation method is the use of the Global Positioning System (GPS) and incorporation of GPS receivers into a mobile unit. However, conventional GPS receivers require a clear view of the skies and signals from at least three satellites, which largely precludes operation in buildings or other RF-shadowed environments. Furthermore, it may take up to several minutes to achieve a location fix on a mobile station. This is too considerable a delay for many applications, especially for emergency services. Finally, there remain questions of cost, size, and power consumption when incorporating GPS receivers into trendy, miniature handsets.
One of the alternatives to GPS positioning is to use time difference of arrival (TDOA) technology in the uplink in which a mobile unit transmits a specific packet to three (3) or more base stations to calculate the location of the mobile unit. The differences in time of arrival at the base stations are utilized in a triangulation method to determine the physical location of the mobile unit.
Although TDOA is currently one of the primary methods used to locate a mobile unit making an emergency call, there are drawbacks with utilizing TDOA. Activation of the location updating procedure is typically responsive to initiation of an emergency call. Prior art systems also employ a fixed period for updating the position of the mobile unit which is independent of any factors related to the application or environment in which the mobile unit is operating. Moreover, this capability is not available in wireless local area networks (WLANs).